Rod on a cylinder

Classical Mechanics Level pending

A thin uniform rod of mass m stays in equilibrium on the top of a fixed horizontal cylinder perpendicular to the axis of the cylinder. Length of the rod is equal to circumference of the cylinder of the cylinder.A beetle of mass 0.2m starts crawling slowly from centre to the end of the rod. As the beetle crawls ,the rod rotates slowly without slipping on the cylinder.find minimum coefficient of friction for this to be possible.


The answer is 0.577.

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1 solution

When the beetle reaches the end of the rod, let the rod rotates through an angle α α from it's initial orientation. Then the normal reaction of the cylinder on the rod is

N = ( m + 0.2 m ) g cos α = 1.2 m g cos α N=(m+0.2m)g\cos α=1.2mg\cos α .

Moment balance equation about the point of contact of the rod and the cylinder gives

m g r α cos α = 0.2 m g ( π r r α ) cos α α = π × 0.2 m m + 0.2 m = π 6 mgrα\cos α=0.2mg(πr-rα)\cos α\Rightarrow α=\dfrac{π\times 0.2m}{m+0.2m}=\dfrac{π}{6} .

For static equilibrium of the rod over the cylinder, the friction force F f F_f is given by F f = ( m + 0.2 m ) g sin α F_f=(m+0.2m)g\sin α . This must be less than or equal to it's limiting value μ s N = μ s ( m + 0.2 m ) g cos α \mu_sN=\mu_s(m+0.2m)g\cos α , where μ s \mu_s is the coefficient of static friction. This implies

μ s tan α \mu_s\geq \tan α .

Substituting the value of α α we get μ s tan π 6 = 1 3 0.5773 \mu_s\geq \tan \dfrac{π}{6}=\dfrac{1}{√3}\approx \boxed {0.5773}

Thanxxx a lott

Priyansh Dutt Sharma - 1 year, 3 months ago

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